In an interview with the Herald last week, Climate Change Minister James Shaw said adults had failed our young "comprehensively" over the past three decades. He understood "completely" why teenagers concerned about inaction on climate change would want to leave their classrooms and take to the streets next Friday. What kind of planet might they have to inherit? Science reporter Jamie Morton looks at five reasons why they deserve to be angry.

A hotter, wilder New Zealand

By the middle of the century, students taking part in this week's climate marches will be in their 40s, and with children of their own.

Average temperatures will be, at best, 0.5C warmer than today - meaning roughly 50 per cent more heatwaves, more droughts and more river floods.

At worst, temperatures will be about 1.5C higher than now.

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"That would mean roughly a tripling of heatwaves and droughts," Victoria University climate scientist Professor James Renwick said.

By the end of the century, assuming greenhouse gas emissions continued to climb close to current levels, many parts of our country will record more than 80 days a year above 25C.

Most places typically only have between 20 and 40 days above that now, yet, already, about 14 elderly people in Auckland and Christchurch die each year when the mercury climbs above 20C.

If global temperatures climb just one, two or three degrees above current levels, that same death rate could rise to 28, 51 and 88 respectively.

With about one in four New Zealanders projected to be 65 and over by 2043 - that's including many people in their 40s today - the problem will be amplified.

The heat isn't the only danger the weather of a warmer world may bring.

In the Auckland region particularly, extreme rainfall will likely increase because a warmer atmosphere carries more moisture.

"Ex-tropical cyclones that approach Auckland in the future may be stronger due to retention of tropical cyclone characteristics farther south than at present," climate scientist Professor Jim Salinger said.

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In the Auckland region particularly, extreme rainfall will likely increase because a warmer atmosphere carries more moisture. Photo / File
In the Auckland region particularly, extreme rainfall will likely increase because a warmer atmosphere carries more moisture. Photo / File

Those impacts will only be compounded by a sea level expected to be 15cm to 30cm higher by 2050.

"This will cause present-day high tide levels to be exceeded more frequently in Auckland."

Low-lying parts of the city – including the CBD, eastern bays, Onehunga, Mangere Bridge, Devonport and Helensville – are the most vulnerable to inundation.

It could hurt health

Beyond the obvious threat of extreme weather, rises in temperature have been shown to fuel myriad knock-on problems for human health.

Hotter days may lead to higher rates of everything from aggression, to heart attacks and strokes.

We already know that when temperatures climb above 18C-20C, hospital and emergency room admissions increase for those with mental health or psychiatric conditions.

Other mental health effects of climate change could range from minimal stress and distress symptoms to clinical disorders such as anxiety, depression, post-traumatic stress and suicidal thoughts.

Studies have shown the routine exposure to news articles like this one are enough to pile more stress on someone's everyday environment.

Research has also pointed to the heightened risk of infectious diseases - including nasty ones we don't have already, like dengue fever and the West Nile virus.

Auckland's temperature is expected to increase by about 0.8C by 2050, but because of uncertainties in greenhouse gas emissions, the range is between 0.5C and 1.3C. Photo / File
Auckland's temperature is expected to increase by about 0.8C by 2050, but because of uncertainties in greenhouse gas emissions, the range is between 0.5C and 1.3C. Photo / File

Scientists have highlighted the threat of new pests like the Asian tiger mosquito - but also warmer water, and heavier yet more infrequent rainfall events, that could set New Zealand up for more Havelock North-scale outbreaks of waterborne illness.

With between 18,000 and 34,000 cases of gastroenteritis per year, the country already has relatively high rates compared with other high-income countries.

But by 2050, the World Health Organisation has suggested New Zealand could expect about one to three more deaths of children because of all causes of diarrhoeal disease as a result of climate change.

The threat of waterborne illnesses - like that which caused a mass-contamination in Havelock North in 2016 - would grow under under climate change. Photo / File
The threat of waterborne illnesses - like that which caused a mass-contamination in Havelock North in 2016 - would grow under under climate change. Photo / File

On top of that, exposure to air pollution could rise with heightened seasonal fire severity, and, pollen season could arrive weeks earlier.

It could hurt the environment

Predicting what will happen to our cherished natural environment is tough – but scientists say we can expect sweeping transformations.

Some particularly native species - among them, tuatara, takahe, rockhopper penguins, little spotted kiwi and Archey's frog - face bleaker chances of adaptation than others.

Alpine environments - refuges to many birds, lizards and invertebrates - will be among the first to suffer as our climate warms.

"The largest change will be a rise of snowlines and continuing loss of glacier ice," Salinger said.

As of 2018 there was 37.5 cubic kilometres of glacier ice - a 40 per cent reduction of that 50 years ago - so another 40 per cent would likely melt, leaving about 20-25 cubic km of glacier ice, he said.

"Ruapehu would lose its ski areas and impacts in the South Island would be that alpine ecosystems would be pressured upwards – this would perhaps lead to extinctions of species in the mountain areas."

In our rivers, streams and lakes, freshwater plants and animals will experience everything from more intense floods and droughts to the knock-on impacts of more irrigation and new pests and weeds.

Some particularly native species - including tuatara - face bleaker chances of adaptation than others. Photo / File
Some particularly native species - including tuatara - face bleaker chances of adaptation than others. Photo / File

On our coasts, estuaries and offshore islands, rising seas will squeeze already fragile habitats against developed land.

Underwater, the toll of acidified oceans could be enormous.

Globally, the oceans' average pH is 8.1, which is 0.1 lower than 250 years ago.

While that might not sound much, a decrease of one pH unit represents a 10-fold increase in the acidity.

The decline in pH is projected to continue in line with the increase in atmospheric carbon dioxide, leading to the most rapid decrease in ocean pH in the past 50 million years.

The effect is associated with decreases in nutrients such as nitrate and phosphate in the surface ocean, where most marine organisms live.

Even small shifts could have big consequences: mussels and paua might struggle to build their carbonate shells, while some fish species could experience changes in behaviour, physiology and habitat distribution.

Niwa scientists estimate perhaps 25 per cent or less of the existing cold water coral locations around New Zealand will be able to sustain their growth by 2100 because of ocean acidification.

An uncertain, unstable world

For the world, Renwick says climate change means further price volatility on staple foods and occasional crop failures in major growing areas like the United States and Eurasia.

There is also a risk of regional conflict and mass migration in other parts of the world, including South Asia and South America.

Under a worst-case scenario, major crop failures could be happening several times per decade – and water and food scarcity could drive the movement of tens of millions of people.

An image of the 2016 Canadian wildfires captured from the International Space Station. This would become a more common occurrence under climate change. Photo / ISS
An image of the 2016 Canadian wildfires captured from the International Space Station. This would become a more common occurrence under climate change. Photo / ISS

Renwick says this means much greater scope for regional warfare over resources, putting millions of lives at risk, or at least displacing populations.

"High temperature extremes would also be making areas in the tropics and near-tropics uninhabitable - or close to it."

His colleague, Victoria University glaciologist Professor Tim Naish, said temperatures and sea levels in the western equatorial and south Pacific were rising faster than anywhere else on Earth.

"There are huge vulnerable populations that will not be able to adapt and will need to migrate - 150 million people world wide live within one metre of current day sea-level."

A sprawling study published late last year highlighted the impacts climate change was already having on the world.

Crop yields have declined in 30 countries because of global warming, raising concerns about malnutrition in some regions, while the risks of dengue fever and malaria have markedly increased around the planet.

Increases in extreme weather events, which in 2017 numbered 712 events resulting in US$326 billion (NZ$480b) in economic losses, were bringing with them injuries and deaths, displacement, post-traumatic stress, and other short and long-term impacts to health.

Another section of the report, authored by New Zealand-based environmental and occupational health expert Professor Tord Kjellstrom, found hotter temperatures cost workers 153 billion productive hours in 2017, in turn affecting family incomes and agricultural output to compound the risks to health.

Around the world, wildfire seasons would grow longer as the globe warmed. Photo / RCMP
Around the world, wildfire seasons would grow longer as the globe warmed. Photo / RCMP

The worst-hit regions were India, southeast Asia, and sub-Saharan Africa, and South America, but Kjellstrom added impacts on New Zealand's labour force would also become more pronounced as heat levels climbed.

It might only get worse

Renwick said the best-case scenario is that warming stopped somewhere between 2050 and 2070.

The worst case is that global temperatures ended up about 4C above present by the close of the century – and sea levels rose 1.5m higher, with at least another 10m locked in across coming centuries.

Major food-producing areas in existence today would be badly affected, much of the tropics would become uninhabitable, and extreme high temperatures would make many mid-latitude areas hard to bear in summer, he said.

Added to that could be quadrupling or more of heavy rainfall and flooding events - with some events well beyond anything observed to date.

Under current projections for 2100, Salinger said significant amounts of road, rail and water infrastructure would be affected in Auckland.

"As well, 4500 residential buildings will be affected during a 1-in-100-year storm tide event with one metre of sea level rise, with a replacement cost of billions of dollars," he said.

"In areas that become even hotter, like the Middle East and North Africa, with temperatures reaching into the 50s, we'd see increased conflict and heatwave fatalities.

"Really bad things could happen with unabated climate change, as we'd be living in a climate so far outside anything humanity has experienced before," Professor James Renwick says. Photo / File

"In New Zealand, there would be marine environment losses of our coastal kelp forests, mortality in our aquaculture farms through heatwaves, and migration of subtropical fish into our fisheries."

Renwick said the bigger increase there was in climate change, the harder it was to judge what would unfold.

"Really bad things could happen with unabated climate change, as we'd be living in a climate so far outside anything humanity has experienced before, and all bets would be off."

Naish said a world 5C warmer could come with mass food shortages, global conflict, economic collapse and population crashes.

"On a positive note, if global warming is severely restricted to somewhere near the Paris Agreement target [restricting future temperature rise within 2C above pre-industrial times], there will be some committed climate change - but the impacts will be significantly less."

The most recent figures show that, if warming continues at the current rate, the Paris Agreement's aspirational threshold of 1.5C will be crossed at some point between 2030 and 2052.

To keep within that mark, carbon dioxide emissions will need to be halved over the next decade, and other greenhouse gases like methane and nitrous oxide will also need to be forced down.

If the 1.5C threshold can be held, the world could escape an extra 10cm of sea level rise, over and above what's already been locked in for this century.

If the world can't hold the line, then the 2C threshold - the ultimate limit the Paris Agreement was built around - could be over-shot only about a decade later.

Given the past trajectory of global emissions of greenhouse gases - nearly a doubling in the past 30 years – climate scientists say turning this picture around requires "far-reaching and unprecedented changes" to every aspect of society.

And, as students around the country will demand on Friday, this bold action has to begin right now.